The present invention relates to a method of reheating metallurgical products, in which solid products, especially steel products, are reheated so as to bring them from a temperature substantially below 400xc2x0 C. to a temperature of at least about 1000xc2x0 C. by passing them through a furnace having an upstream zone in which the said products are preheated and a downstream zone in which the said products are brought to their final temperature on leaving the furnace, the downstream zone of the furnace being fitted with burners, at least some of which operate with an oxidizer which is air, the smoke (flue gases) generated by these burners flowing as a countercurrent to the products and preheating these products in the upstream preheating zone (the terms xe2x80x9csmokexe2x80x9d or xe2x80x9cflue gasesxe2x80x9d are hereafter used with the same meaning).
Reheat furnaces are used in the steel industry to reheat steel products coming especially from continuous casting and to bring them to the rolling temperature which is around 1000to 1300xc2x0 C.
Furnaces of this type usually consist of several successive zones. Starting from the charge end (in the direction in which the products run through the furnace), these successive zones are the upstream zone called the flue gases exhaust (or recovery) zone in which the thermal energy of the flue gases, which is produced downstream in the furnace and which flows as a countercurrent to the products to be reheated, is recovered in order to start to preheat these products.
This preheating zone is followed by one or more heating zones, the furnace terminating in an equalization (or soaking) zone which serves to ensure that the temperature of the product leaving the furnace is homogeneous. Burners may be preferably installed on each side of the product which travels from the preheating zone to the end of the heating zones. Such burners may also be placed in the roof of the furnace (radiant roof case) or else in recesses depending on the width of the furnace.
While the products are passing through the various successive zones of the reheat furnace, the temperature of the product at the surface and inside it progressively increases. Owing to the characteristic times for thermal conduction, especially in steel, there is a temperature difference between the top side of the product and the underside or else between the top side of the product and the core of the product. Controlling these thermal inhomogeneities is an important aspect of the invention.
This problem of obtaining temperature homogeneity of the product is all the greater the more limited the thermal power that can be injected into a reheat furnace. There may be several reasons for this limitation: the limited volume of smoke, temperature of one or more zones of the furnace at maximum, temperature at the inlet of the energy recuperator at maximum, etc. In all cases, the limitation in injected thermal power results in a limitation in the energy transferred to the product and therefore to thermal inhomogeneities throughout the mass of the product appearing or increasing. In order to provide a better explanation of the problem facing a person skilled in the art, FIG. 1 shows the curve of how the temperature difference xcex94T (defined below) varies as the product is being reheated.
For a furnace in which the products rest on the hearth, the temperature difference xcex94T will be the difference between the temperature of the top side of the product exposed to the radiation of the furnace and the temperature of the underside of the product in contact with the hearth.
For a walking beam furnace, that is to say one in which the hot gases of the furnace circulate all around the product, the temperature difference xcex94T will be the difference between the surface temperature and the core temperature of the product.
In FIG. 1, the position of the product in the furnace has been plotted on the x-axis and the xcex94T value on the y-axis. The initial temperature difference (xcex94Tinit) may be zero, when the product is at room temperature at the charge end of the furnace, or non-zero in the case of products whose temperature has not yet become homogeneous again, for example in the case of the treatment of metallurgical products shortly after their production. In FIG. 1, X represents the position of the product in the furnace, 0 being the charge end where the products enter the furnace, while XB is the discharge end or exit of the furnace.
The curve (C) showing the variation of xcex94T as a function of X in FIG. 1 has a point A where the parameter xcex94T reaches a maximum (xcex94Tmax), a point D where the parameter xcex94T has a value xcex94Tinit, which is the value of xcex94T of the product at the charge end and a point B where the parameter xcex94T has a value xcex94Tfinal of the product at the exit (discharge end) of the furnace.
Somewhere in the middle of the furnace, at the point XA, the temperature difference xcex94T reaches its maximum (xcex94Tmax). This xcex94Tmax value must be as small as possible, since a large temperature difference is equivalent to deformations (bending) of the product which may result in the product being damaged or in the furnace not being able to be operated or in the product leaving the furnace not being able to be rolled. Thus, in certain furnaces the operators must limit the power of the furnace and/or its production in order to avoid the appearance of excessively large temperature differences xcex94T. This is a major drawback for an industrialist.
It is therefore a first object of the present invention to prevent the appearance of excessively large temperature differences in the product throughout the time it is passing through the furnace.
FIG. 2 illustrates the relationship between the temperature difference xcex94T and the sag, that is to say the vertical deformation, of the product during its passage through the furnace.
This FIG. 2 shows the curve (C), as in FIG. 1, and a curve (F) which represents the vertical deformation of the product as a function of X. It may be seen that the maximum deformation corresponds approximately to the maximum xcex94T (xcex94Tmax for X=XA).
Moreover, it has been shown that another important parameter is the temperature difference xcex94Tfinal at the exit of the furnace. Ideally, xcex94Tfinal should be zero at the exit (discharge end) of the furnace. In practice, a certain temperature difference xcex94Tfinal is tolerated, but it must not exceed about 100xc2x0 C. in the case of billets and 200xc2x0 C. in the case of slabs and blooms. This is because a large temperature difference causes rolling difficulties which may result in mechanical hitches in certain stands of the rolling mill. In addition, any temperature inequality is manifested by a reduction in quality of the finished product.
It is also an object of the present invention to reduce xcex94Tfinal of a product exiting a reheat furnace without increasing the consumption of energy in the furnace.
The article entitled xe2x80x9cEfficient operation of continuous reheat furnaces through oxygen optimization of combustion systemxe2x80x9d by G. Gitman, T. Wechler and B. Levinson, published in the journal Industrial Heating, describes various systems for reheating metallurgical products and suggests the use of oxy-fuel burners instead of the usual air-fuel burners, so as to increase the energy transfer to the said products and maintain or even increase the xcex94Tmax of these products, as illustrated in FIG. 7 of that article.
Contrary to the method described in the above article, the method according to the invention consists of the use of burners whose oxidizer has an oxygen concentration greater than 21 vol % and less than or equal to 100 vol % (hereafter called xe2x80x9coxy-burnerxe2x80x9d), these burners being installed in the furnace so that they are the first burners xe2x80x9cseenxe2x80x9d by the products to be treated as they progress through the furnace, after the latter has been charged therewith. The preheating zone formed by these oxy-burners is therefore the first preheating zone of the furnace. In the case of new furnaces, the invention therefore consists in placing oxy-burners in that zone of the furnace where the first burners have to be placed (xe2x80x9cfirstxe2x80x9d is understood to mean with regard to the direction in which the metallurgical product runs through the furnace).
The method according to the invention is characterized in that at least one burner is placed in the upstream preheating zone of the furnace, this burner being fed with an oxidizer and a fuel, the oxidizer containing more than 21 vol % and preferably more than 30 vol % oxygen. The oxidizer and fuel may be fed into the burner either by separate injection (injectors opening into the furnace) or by coaxial injection (coaxial multitube burner) or by premixing the oxidizer with the fuel before injection into the burner and then into the furnace. These various injection techniques are well known per se to those skilled in the art.
In the case of the modification of an existing furnace, the invention may comprise two implementation variants. The first variant consists in creating a new furnace zone having oxy-burners.
To do this, the oxy-burners are installed in a zone of the furnace which originally did not have any burners. By way of example, this may consist in installing oxy-burners at the end of the furnace zone called the recovery zone, just before the first heating zone which (normally has air-fuel burners).
The second variant consists in converting an existing zone, that is to say all or some of the air-fuel burners are removed from an existing preheating zone to be replaced with oxy-burners installed in the same zone.
The two variants of the above solution in existing furnaces may be implemented separately or in combination.
According to a third variant, the method according to the invention is characterized in that the proportion of oxygen in the oxidizer injected into the said oxy-fuel burner depends on the preheating temperature of the existing air-fuel burners, the proportion of oxygen being chosen so that the thermal efficiency of the said oxy-fuel burner is greater than the thermal efficiency of the existing air-fuel burners.
According to a fourth variant, the method according to the invention is characterized in that the proportion of oxygen in the oxidizer injected into the said burner is greater than or equal to 88 vol %, preferably greater than or equal to 95 vol %.
According to a fifth variant, the method according to the invention is characterized in that the oxidizer delivered to the said at least one burner is a mixture of air and industrially pure oxygen.
According to a sixth variant, the method according to the invention is characterized in that the oxidizer delivered to the said at least one burner is a mixture of air and oxygen coming from a VSA (Vacuum Swing Adsorption) system well known to those skilled in the art.
Finally, according to another aspect of the invention, the method according to the invention is characterized in that the oxidizer injected into the said at least one burner includes from 1 to 5 vol % of argon. Since the molar mass and the density of argon are higher than those of oxygen, the presence of argon in the oxygen-containing oxidizer makes it possible to increase the momentum of the flame. This increase in momentum will give a more stable flame, less sensitive to transverse flows, closer to the metallurgical product to be reheated, and it will therefore consequently provide more effective and more homogeneous heating of the product to be reheated.